In stereotactic radiosurgery, a target is localized relative to an external reference frame attached to the head. The target is then irradiated from a wide range of orientations. Localization and irradiation are conventionally applied to a recumbent patient. A radiation source, either for imaging or for treatment, rotates in a vertical plane (i.e., about a horizontal rotation axis) about the patient. The patient's couch may be translated for imaging or rotated about a vertical rotational axis for radiation treatment. There is no relative motion between patient and couch. Repeatable source rotations in conjunction with a series of couch orientations result in irradiation through longitudinal arcs relative to the head (e.g., U.S. Pat. Nos. 5,160,337 and 5,189,687). The method of head immobilization depends on the treatment: frame-attached pins are inserted into the skull for a single-fraction radiosurgery, while detachable immobilizers like a biting block and/or a face mask are used for fractionated radiotherapy. The rotating gantry and the couch are cumbersome, expensive and mechanically imperfect due to difficulties in maintaining intersection of the respective rotational axes with each other and with the beam.
Upright head radiotherapy is taught in U.S. Pat. No. 5,250,019. The patient is rotated about a vertical rotational axis while a radiation beam is incremented through elevation angles that intersect the rotational axis at an isocenter. The beam trajectories form concentric cones having a joint apex at the isocenter where the target is placed. However, intersecting the turntable rotational axis with the isocenter presents a mechanical difficulty.